The Roman Empire (Imperium Romanum in Latin) consists of three periods: the Principate, the Dominate and the Eastern Roman Empire (also known as the Byzantine Empire). The Roman Empire initially tried to give the impression of a democratic form of government but it was in reality autocratic. Consuls continued to be elected, the tribunes continued to put forth new legislation (on behalf of the people) and senators would still have debates in the curia. However, all final decisions would be made by the Emperor and the Emperor had full control of the military.
It does not mean though that the Empire was not prosperous. One should bear in mind that the Roman Republic had plenty of civil wars and revolts and that life during the Republic was not always easy for the average Roman citizen. The first two centuries of the Empire were extremely stable. There were very few revolts in the provinces, with the exception of the Jewish-Roman wars. Overall, these were good times to live in Rome. The economy was thriving, trade in the Mediterranean was flourishing. The Empire’s territory stretched from Northern Europe to North Africa and the Middle East.
The third century saw civil wars breaking out throughout the Empire, foreign invasions, hyperinflation and economic depression, and even pandemics. This led to the end of the Principate and to the beginning of the Dominate. The Dominate was initially a true republic and more democratic than the Principate ever was. The capital moved from Rome to Mediolanum (today's Milan).
At the beginning of the 4th century, Emperor Constantine moved the capital to Constantinople (initially named Nova Roma and made Christianity the official religion of the Empire. Later in the Dominate, Theodosius (reign: 379-395 A.D.) built massive impenetrable walls around Constantinople and divided the Empire between a Latin speaking West and a Greek speaking East.
The Latin West central government collapsed in the 5th century when barbarian forces invaded Ravenna the then capital of the Western Empire. The East however remained in what would be known as the Eastern Roman Empire.
The Roman Empire shaped European culture and traditions as the barbarians had kept many of the Roman customs. However, what used to be the Western Empire became very divided. Various kingdoms sprang up. Commerce declined. Roads stopped being maintained. Walls would be built around cities. Aqueducts fell into disrepair or were destroyed. Water was not as easily accessible (and people would not wash as often). After the 5th century, the Eastern Roman Empire continued to resemble a Roman society however with a strong Christian influence.
You can click on any of the links below to read about each period of the Roman Empire:
The Roman Principate (27 B.C. – 284 A.D.)
The Roman Dominate (284 - 476 A.D.)
The Eastern Roman Empire (476 - 1453 A.D.)